The strength of line pipes for use in the transportation of natural gas and crude oil has been increasing year by year to improve transportation efficiency by means of high-pressure transportation or on-site welding efficiency by means of thickness reduction. The demand for line pipes having a tensile strength above 800 MPa is being met.
While line pipes are generally seam-welded by submerged arc welding, seam welding of high-strength steel line pipes having a tensile strength above 800 MPa may cause weld metal cold cracking. It is known that the welding of high-strength steel of HT 80 or more (above 80 kg/mm2(780 MPa)) may cause cold cracking. In general, cold cracking is prevented by a decrease in the hydrogen content of welding consumables and heat treatment for hydrogen diffusion (decrease of diffusible hydrogen), such as preheating, post heating, or interpass temperature management.
For example, Japanese Patent No. 3726721 discloses a method for preventing weld cracking, which includes defining the time period from welding to cooling to 100° C. and performing post heating. However, preheating and post heating in the seam welding of line pipes greatly decrease the production efficiency of line pipes. In industrial production of high-strength line pipes, therefore, it is important to prevent cold cracking of seam weld metal without performing preheating or post heating.
To prevent cold cracking, for example, Japanese Unexamined Patent Application Publication No. 2002-115032 proposes a method for preventing cold cracking by setting the retained austenite content of internal weld metal at 1% or more. However, with a weld metal having a strength as high as 800 MPa or more, even the inclusion of 1% or more of retained austenite sometimes cannot prevent cracking.
Japanese Patent No. 3582461 proposes a method for preventing weld metal cold cracking by setting the Ms point of the weld metal at 375° C. or less and thereby inducing tensile stress relaxation (decrease of residual tensile stress) due to transformation expansion. However, because this method principally aims to decrease the Ms point of weld metal, the proportion of a martensite structure, which is susceptible to cold cracking, increases. Decreasing the Ms point is therefore not always effective and may decrease low-temperature toughness.
To increase the weld metal strength to 800 MPa or more, it is essential to exploit the martensite structure. For example, Japanese Patent No. 3519966 discloses a low-temperature transformation microstructure, such as martensite and bainite, for higher strength. An internal weld metal having such a martensite structure recovers toughness owing to a tempering effect produced by welding heat input to an external surface. In the case that the location of the notch in a Charpy impact test specimen includes an overlap portion of the internal and external surfaces, therefore, it is relatively easy to secure weld metal toughness. However, an external weld metal is not tempered by other welding heat and includes an untempered structure (so-called fresh martensite structure). The fresh martensite is known to be of low toughness and be susceptible to hydrogen embrittlement. Thus, ensuring the toughness of unheated external weld metal becomes an issue.
It could therefore be helpful to provide a high-strength steel pipe that has a tensile strength of 800 MPa or more and includes a weld metal having high cold-cracking resistance and high low-temperature toughness.